The present invention relates generally to an electromagnetic acoustic transducer (EMAT) which is suitable for applications for detecting surface and internal damages and residual stress of an electrically conductive material, measuring the characteristics of the material (such as elastic modules, and attenuation coefficient), and detecting damages and deterioration in the interior of the material due to load, heat, and so on, and to an inspection system with the electromagnetic acoustic resonance which is a combination of resonant technique and non-contacting EMAT, and which improves fairly the signal to noise ration (S/N) and compensates for the weak coupling efficiency of the EMAT.
FIG. 1 is a diagram for explaining the measuring principles of an inspection system using an EMAT. The EMAT transducer consists of a pair of permanent magnets 3a, 3b which have the opposite magnetization direction normal to the objects 2 surface, that is a conductive material, through a spacer 3c, and an elongated electrically spiral elongated coil unit 5 positioned below the permanent magnets 3a, 3b. The permanent magnets may be replaced with electromagnets which can form a similar magnetic field.
The permanent magnets 3a, 3b generate a static magnetic field 4 in the depth direction of the object 2. When the coil unit 5 is supplied with a rf burst current 7 from a controller or processor 18, an eddy current 8 is generated on the surface of the object 2 in the direction reverse to that of the current 7. Then, the eddy current 8 and the static magnetic field 4 interact to generate a Lorentz force 9 in accordance with the so-called Fleming's law. The Lorentz force 9 acts on internal free electrons of the object 2 to cause collision with ions and so on, to induce movements thereto in a direction perpendicular to the directions of the static magnetic field 4 and the rf burst current 7 in the interior of the object 2, and to generate ultrasonic shear waves 10.
As the ultrasonic shear waves 10 travel in a direction indicated by an arrow 11, the ultrasonic shear waves 10 reflect on the upper and lower surfaces of the object 2 and interior damages, defects, grain boundaries, changes of material's structure and so on therein, and thereby return in a direction indicated by an arrow 12. When the reflected ultrasonic shear waves 10 return to the vicinity of the upper surface, a force 13 is generated. Then, an eddy current 14 is generated by an interaction of the force 13 and the static magnetic field 4. The eddy current 14 is detected by the spiral elongated coil unit 5 and the detected current is amplified by a pre-amplifier 16 and a main amplifier 17, and then sent to the controller 18. The controller 18 analyzes the received current to measure internal damages, defects, grain boundaries, changes of material's structure and so on of the object 2.
FIGS. 2(a) and 2(b) schematically illustrate a conventional EMAT composed of a pair of permanent magnets 3a, 3b and a spiral elongated coil unit 5, which is usable in an inspection system as shown in FIG. 1. FIG. 2(a) is a top plan view of the transducer and FIG. 2(b) is a cross-sectional view taken along a line A--A in FIG. 2(a). The spiral elongated coil unit 5 includes a transmitter coil 51a and a receiver reception coil 51b which are fabricated by manually winding a pair of enamel wires closely to each other and by solidifying the wound enamel wires with a resin. The spiral elongated coil unit 5 further includes a protection film 52 having a thickness of approximately 0.1 mm adhered thereto to support and protect the coils 51a, 51b. By the protection film, when a ferromagnetic object to be inspected is placed, the spiral elongated coil unit 5 may be prevented from damages caused by magnetic forces generated by the permanent magnets 3a, 3b and the ferromagnetic object. A space or spacer 3c is interposed between the permanent magnets 3a, 3b which have the oppose magnetization directions.
The conventional EMAT shown in FIG. 2 requires the protection film 52 adhered on the surface of the spiral elongated coil unit 5 in order to prevent the unit 5 from being damaged. Also, since the receiver and transmitter coils 51a, 51b are arranged on the same surface adjacent to each other, the spiral elongated coil unit 5 inevitably has a large flat size. Further, since the coils 51a, 51b is made by manually winding enamel wires, dispersions in the winding of wires are caused, and thus resulting in wide variations in the performance of finished transducers. Accordingly, it may not provide a stable quality.
As to other conventional spiral elongated coil units for generation and detection of ultrasonic waves which EMAT, the following prior arts have been provided, for example: Japanese Patent Public Disclosure (Laid-Open) No. 53-1078 (1978) discloses a structure having double spiral coils arranged on the opposing sides of an insulation substrate for providing separate transmission and reception coils. However, although the structure has the separate transmission and reception coils, the two coils are not provided with a common ground, thus exhibiting a low S/N ratio and a small gain.
Japanese Patent Public Disclosure (Laid-Open) No. 62-277555 (1987) discloses a structure having a single parallel coil on one surface of an insulation substrate so as to use the coil for both transmission and reception. In this prior structure, since the single parallel coil arranged on one surface of the insulation substrate is used for both transmission and reception, the S/N ratio is low and the gain is small.
Japanese Patent Public Disclosure (Laid-Open) No. 53-23066 (1978) discloses parallel coils formed on one surface of an insulation substrate in an "8" shape (i.e., two coils winding in opposite directions) using printing techniques. In this prior art, since the 8-shaped parallel coil is formed on one surface of the insulation substrate, the entire size becomes larger. In addition thereto, since the two coils are not provided with a common ground, the S/N ratio is low and the gain is small.
FIG. 3 shows another conventional inspection system (excluding a controller and amplifier means) for detecting surface and internal damages and residual stress of an electrically conductive material, measuring the characteristics of the material (such as elastic modules, and attenuation coefficient), and detecting damages and deterioration in the interior of the material due to load, heat, and so on. In the system, an EMAT comprises a meander coil unit 6 positioned on an inspection object 2 of a flat steel plate, and a permanent magnet 3, which generates a static magnetic field or bias magnetic field, positioned above the meander coil unit 6.
When the meander coil unit 6 of the EMAT is applied with a rf burst current from a controller (not shown), rf burst currents flow on the surface of the object beneath parallel lines of the meander coil unit 6 in alternate directions. This results in generating Lorentz forces 9 in reverse directions to each other, which in turn generate ultrasonic waves 10 in the direction perpendicular to the parallel lines of the meander coil unit 6 on the object 2. When the ultrasonic waves (SO wave) 10 reflect on damages, defects, grain boundaries, structural changes, and so on on the surface and in the interior of the object 2, and reflect on the opposing surface of the object 2. When the reflected waves reach the vicinity of the surface of the object 2, a force is generated. Then, a current is generated by an interaction of this force and the static magnetic field, and detected by the meander coil unit 6 to measure damages, defects, grain boundaries, structural changes, and so on on the surface and in the interior of the object 2.
The conventional meander coil type EMAT shown in FIG. 3 uses a single coil in the coil unit both as a transmitter coil for generating ultrasonic waves and as a receiver coil having a detecting function. For this reason, the transducer exhibits a low S/N ratio and a small gain. In addition thereto, since the meander coil unit 6 is constituted to be flat and not to be flexible, the transducer cannot be utilized for a cylindrical object to measure damages, defects, grain boundaries, organizational changes, and so on on the surface and in the interior thereof.
In a prior art, a fatigue life and a remaining life of a metal material at an initial stage of its fatigue process is predicted by using a piezoelectric ultrasonic transducer, contacting ultrasonic transducer. As illustrated in FIG. 4(a), a piezoelectric ultrasonic transducer (P.Z.T) 21 vibrates to generate ultrasonic waves which mechanically propagate to an object 2 of a metallic material through a protective film 22 and an acoustic couplant 23 during transmitting. During receiving, the process reverse to the transmission is performed, as illustrated in FIG. 4(b), wherein vibrations of the object 2 mechanically propagate through the acoustic couplant 23 and the protective film 22 to the piezoelectric ultrasonic transducer 21 which transduces the mechanical vibrations into ultrasonic waves. The ultrasonic waves are processed and analyzed by a controller (not shown) to predict fatigue life and remaining life of the object 2.
In the above conventional method of predicting a fatigue life and a remaining life, the ultrasonic waves often scatter due to reflections on interfaces (those are an interface between the piezoelectric ultrasonic transducer 21 and the protective film 22, an interface between the protective film 22 and the acoustic couplant 23, and an interface between the acoustic couplant 23 and the object 2) during its propagation process. Due to the scattering or absorbing ultrasonic waves, energy, which would otherwise be received by the piezoelectric ultrasonic transducer 21, leaks, and a phase change of the waves occurs when they reflect, thus resulting in disturbed signals.
Also disadvantageously, the characteristic and thickness of the acoustic couplant 23 are likely to vary due to change of temperature and pressing force applied thereto, so that measured values may often widely vary. In addition, measured values also fairly vary unless a measuring surface of an object is finely finished. For these reasons, a relative comparison with an initial value before fatigue has been used in a fatigue life and remaining life prediction method relying on attenuation of ultrasonic waves detected by the piezoelectric ultrasonic transducer 21. Further, it is difficult to measure the absolute attenuation of the ultrasonic waves to predict the lifetime of the object.
A prior document states that in a measurement of attenuation characteristics during a fatigue developing process using a piezoelectric ultrasonic transducer, the attenuation increases linearly at first and rises rapidly at about 70-80% of fatigue life, and the attenuation abruptly shown in FIG. 5. Thus, if remaining life is evaluated based on a point at which the attenuation abruptly increases, while monitoring the fatigue process, it is necessary to monitor the fatigue process up to a rather later time of the lifetime.